netfilter: nf_conncount: Early exit for garbage collection

This patch is originally from Florian Westphal.

We use an extra function with early exit for garbage collection.
It is not necessary to traverse the full list for every node since
it is enough to zap a couple of entries for garbage collection.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>